Sister Helen Prejean, a death penalty opponent, speaks during a news conference for Richard Glossip in Oklahoma on Monday. Glossip is scheduled to be executed on Wednesday.
Photograph: Sue Ogrocki/AP

An Oklahoma district attorney has denounced claims that a death row inmate scheduled to die this week is innocent as “a bullshit PR campaign”, saying that legal representatives for the condemned prisoner have failed to produce any new documentation supporting their argument.

Attorneys for Richard Glossip, who are appealing to Governor Mary Fallin for a 60-day stay of execution, held a press conference at the Oklahoma state capitol on Monday to present new information they claim supports efforts to exonerate him. Glossip – whose high-profile supporters include former senators, the actor Susan Sarandon and anti-death penalty activist Sister Helen Prejean – is scheduled to die by lethal injection on Wednesday for the 1997 murder of his boss, Barry Van Treese.

Is Oklahoma about to execute an innocent man? Richard Glossip's supporters say yes

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“All they are trying to do is abolish the death penalty in the state of Oklahoma and this country by spreading a bunch of garbage,” Oklahoma County district attorney David Prater told the local Fox 25 TV station after attending Monday’s press conference. Prater, who did not prosecute Glossip, told Fox 25 he believes Glossip is guilty and said the man’s attorneys have refused to hand over to him any of the evidence they presented on Monday.

The evidence produced on Monday by defense attorney Don Knight includes a sworn affidavit from a man who claimed to have heard a confession from the man on whose testimony Glossip was convicted, Justin Sneed. It also includes a statement from another man who claims the prosecution’s argument that Glossip controlled Sneed was false.

Sneed is currently serving a sentence of life without parole at Joseph Harp Correctional Center, a medium-security prison in Lexington, Oklahoma. Glossip was convicted twice of Van Treese’s death, once in 1998 and again in 2004 after his first verdict was thrown out for ineffective council.

Richard Allan Barrett, a man who claims to have been a friend of Glossip’s brother, Bobby, and a drug dealer at the Best Budget Inn the year Van Treese was killed, claimed Sneed had regularly used robbery to finance a methamphetamine addiction that was more severe than prosecutors had admitted.

“Bobby Glossip told me to always keep my car locked when I was at the hotel,” Barrett said. “Later I learned this was because the maintenance man (Sneed) broke into cars at the motel parking lot and stole items from the car.”

Barrett said that Sneed traded items – like car stereos and radar detectors in exchange for drugs – he believes were stolen.

“I was present when Justin Sneed told Bobby Glossip that he had taken these items from occupied rooms at the motel and cars in the parking lot of the motel and other businesses near the hotel.”

Barrett also said he “saw nothing to make me believe Justin Sneed was controlled by Richard Glossip”, or that Glossip was aware of Sneed’s alleged heists.

Barrett’s statement appears to suggest that Sneed had robbed people before, and supports the claim that Van Treese’s death was not a premeditated homicide orchestrated by Glossip, as prosecutors had argued, but the result of a robbery gone wrong.

A separate affidavit signed by Dr Richard A Leo, a professor at the San Francisco University School of Law, said Sneed’s implication of Glossip was coerced by police and therefore unreliable.

“It has been well-documented in the empirical social science research literature that hundreds of innocent suspects have confessed during police interrogation to crimes (often very serious crimes such as murder and rape) that it was later objectively proven they did not commit,” Leo wrote.

Leo said that investigators used false claims and intimidation to lead Sneed to believe that implicating Glossip would save him from a harsher sentencing.

Like Glossip’s attorneys, Leo stressed that Sneed told eight different versions of the murder, four of which were given during his interrogation. At various points during his police interrogation a week after the murder, Sneed, then 19, said he didn’t know Van Treese, then that he didn’t kill Van Treese, then that he had killed him accidentally, and then that he had killed him intentionally, under Glossip’s instruction. Eventually, Sneed agreed to a plea deal in which he would testify against Glossip to save himself from the death penalty.

“These accounts do not fit with one another, or the crime scene facts, and thus are one indicator of the potential unreliability of Sneed’s statements implicating Richard Glossip in this case,” Leo wrote.

Another affidavit produced on Monday was from Michael Scott, who spent time in the cell across from Sneed and claims to have heard Sneed admit to making up testimony.